Problem: What is $42\%$ of $900$ ?
Explanation: Having $42\%$ of something means that you get $42$ out of every $100$ We can set up a proportion to find out what number is $42\%$ of $900$ $ \dfrac{{\text{percent}}}{100} = \dfrac{{\text{part}}}{{\text{whole}}}$ Which things do we know, and what are we trying to find? We know the ${\text{percent}}$ is $42$ . Is $900$ the ${\text{part}}$ or the ${\text{whole}}$ The $900$ is the ${\text{whole}}$ . We are trying to find the ${\text{part}}$ that makes up $42\%$ of it: $ \dfrac{{42}}{100} = \dfrac{{\text{part}}}{{900}}$ If we multiply the denominator of the fraction on the left by $9$ , it will be the same denominator of the fraction on the right. To keep things equal, let's also multiply the numerator on the left by $9$ $ \dfrac{{42} \times 9}{100 \times 9} = \dfrac{{\text{part}}}{{900}}$ $ \dfrac{{378}}{900} = \dfrac{{\text{part}}}{{900}}$ $ {378} = {\text{part}}$ So $378$ is $42\%$ of $900$.